For those skilled in the WECS technical field, there are always many problems to be solved. For example, how to improve the wind energy utilization efficiency, and particularly how to acquire the wind energy utilization efficiency as high as possible in a specific swept area; how to make the WECS more compact in size; how to operate the WECS more safely; and how to reduce noise pollution of the WECS.
The existing well-known common three-bladed horizontal-axis-rotor WECS is the most universal and representative WECS. However, it still has some drawbacks, such as it has a lower average wind energy utilization coefficient. Although the highest value is 0.593 according to the Betts Limit theory, this value can never be reached. The most advanced current technical index is about 0.4, but this value can be available only simultaneously under various optimum conditions, such as the wind speed is ideal at about 12 m/s, and the tip-speed ratio is close to 5. In an actual complicated and volatile operation process, the wind energy utilization efficiency is far below such a level of 0.4. Moreover, a drawback of the three-bladed horizontal-axis-rotor WECS is that the tip-speed ratio of the horizontal-axis-rotor turbine is generally about 4-7; under such high speed conditions, the blade cutting airstream will produce a very high aerodynamic noise, and meanwhile a lot of birds can hardly survive. Therefore, some countries begin to limit and even forbid practical application of this kind of WECS. Furthermore, with a very high turbine turning speed of the running horizontal-axis-rotor WECS, a large centrifugal force under very strong wind conditions will usually result in breakup and damage of the rotor, even a “runaway” injury accident. That is, the horizontal-axis-rotor WECS is not safe enough. Besides, it is not suitable to be mounted at such places as roofs and high buildings in a city, which also limits its application location. The reasons as described above indicate that the WECS must be continuously innovated and improved to further meet the requirements of customers.
However, all the various disclosed technical approaches can hardly solve the above problems ideally, either for the horizontal-axis-rotor WECS or for the vertical-axis-rotor WECS.
An explanation will be made below with reference to the relevant background of knowledge and technology. Some terms and relevant knowledge and principles are mentioned in many places of the present specification, such as “Bernoulli principle”, “Venturi-tube effect”, “blade airfoil”, “tip-speed ratio”, “relation between tip-speed ratio and wind energy conversion efficiency”, and “wind energy capturing efficiency”. This knowledge and principles are the basic concepts and knowledge in the fields of aerodynamics and wind power generation, and will thus not be described here in detail.